Photodynamic therapy (PDT), which is the interaction of light with hematoporphyrin derivative (HDP, Photofrin II), is in clinical use in several areas of application. Currently, PDT is being used clinically for a variety of skin lesions and for bronchial obstructions and is being investigated in the following: cancer of the prostate (beagles); carcinoma in situ of the bladder and urinary tract (foxhounds); and pulmonary metastatic disease (nude mice). Specially designed fiber optic probes and introducers were manufactured for all cases, and calibrated in terms of irradiance delivered to the tissue. Photosensitized skin is a side effect of PDT, and the possibility of using agents to protect against this photosensitivity was addressed. In particular, hydrochlorothorazide appears to have a pronounced effect on reducing skin photosensitivity in nude mice models. Dosimetry for PDT is complex due to the variable irradiation levels within the tissue, sequestering of HDP, and tissue oxygenation. To address the first of these points and to permit light distribution within tissue to be formulated, light emanating from tissue was measured as a function of radial distance from the point of injection of the light into the tissue.